This invention relates to the field of measuring constituent components of gas mixtures, and more particularly, to measuring the concentration of nitrogen in a gas mixture.
The concentration of nitrogen in a gas mixture is difficult to measure directly. Nitrogen has low infrared absorption characteristics, which makes infrared sensing methods difficult. Also, it is chemically inert, which makes electrochemical sensing methods difficult.
Yet, it is often desired to determine the amount of nitrogen in a particular gas mixture. For example, in a natural gas, nitrogen is a diluent and the amount of nitrogen affects heating value. Experimentation has indicated that a plus or minus shift of 0.075 mole % in nitrogen concentration will produce a plus or minus shift of 1.0 BTU/SCF in standard volumetric heating value.
U.S. patent application Ser. No. 09/550,431, entitled xe2x80x9cSystem and Method to Determine Thermophysical Properties of a Multi-Component Gasxe2x80x9d and U.S. patent application Ser. No. 10/237,492 entitled xe2x80x9cA System and Method to Determine Thermophysical Properties of a Multi-Component Gas at Arbitrary Temperature and Pressurexe2x80x9d, to K. Behring and T. Morrow, describe inferential methods for deriving the heating value of natural gas. These methods developed from correlation studies of the heating value of a large number of representative samples of natural gas and three independent physical parameters associated with the gas, namely, the speed of sound at specified pressure and temperature and the fractional concentrations of two diluent gas components (carbon dioxide and molecular nitrogen).
U.S. patent application Ser. No. 09/550,431 further describes various methods for determining the amount of molecular nitrogen in a natural gas sample. These methods are inferential in nature in that the concentration of nitrogen in representative natural gas mixtures, like the gas heating value, is correlated with the speed of sound and the carbon dioxide concentration at two independent thermodynamic states. One inferential nitrogen measurement technique is further described in U.S. patent application Ser. No. 10/371,419 entitled xe2x80x9cIndirect Measurement of Nitrogen in a Multi-Component Natural Gas by Heating the Gasxe2x80x9d, a continuation-in-part of U.S. patent application Ser. No. 09/550,431.